The present invention relates to a method of transforming an analog signal read out by an image sensor within a reading section, a facsimile device, or the like, into a binary signal.
In a reading apparatus in which real picture information is read by a linear image sensor, the transformation of an analog signal into a binary signal is effected for every main scanning line in accordance with the light and dark (i.e. white and black) portions of an original document.
FIG. 1 is a block diagram explaining the principle of the binary transformation of an analog image signal in a conventional reading apparatus. An analog picture signal 2 produced from an image sensor 1 is applied to an automatic background control circuit 3. The automatic background control circuit 3 is a type of automatic gain control (AGC) circuit which serves to cause the levels of the background (i.e. white) regions to coincide with each other in each image. An analog picture signal 4 from the background control circuit 3 is compared by a comparator 5 with a threshold voltage V.sub.ref which is a reference value for the binary transformation. As a result of this comparison, a binary-transformed signal 6 is produced which corresponds to light and dark portions of the original document.
FIG. 2 shows a resolution characteristic in such a binary transformation method. In FIG. 2, the ordinate represents the modulation transfer function (MTF) and the abscissa represents the line density. As seen in FIG. 2, as the line density increases, the contrast between the white portion and the black portion of a picture image decreases. This decrease is due to the respective resolution characteristics of the image sensor and its associated focusing lens (which focuses the light image onto the image sensor). As a result, it becomes impossible to divide the picture signal into two values, i.e., white and black, which correspond to the light and dark portions of an original document.
The decrease in contrast described above will be explained in more detail with reference to FIGS. 3A-5. In each of FIGS. 3A and 3B, we shall assume that a pixel P.sub.i,n disposed at the center of a plurality of aligned pixels is the pixel of an original document which corresponds to a bit thereof which is to be subjected to binary transformation. Pixels P.sub.i,n-1 and P.sub.i,n+1 are on the left and right sides, respectively, of pixel P.sub.i,n on the same scanning line i. Specifically, pixel P.sub.i,n-1 corresponds to a bit on the document which has been subjected to binary transformation immediately before pixel P.sub.i,n and pixel P.sub.i,n+1 corresponds to a bit of the document which will be subjected to binary transformation immediately after pixel P.sub.i,n. Correspondingly, pixels P.sub.i,n-2 and P.sub.i,n+2 on the left and right sides, respectively, of pixel P.sub.i,n are pixels on the original document corresponding to a bit which was subject to binary transformation one bit before pixel P.sub.i,n and to a bit which will be subject to binary transformation one bit after pixel P.sub.i,n, respectively, on the same scanning line i.
In FIG. 3A, the central pixel P.sub.i,n is black and the other ones are white. When the image sensor reads the central pixel, it also reads the picture information with respect to the adjacent pixels P.sub.i,n-1 and P.sub.i,n+1 in combination (due to poor resolution). In this case, as shown in FIG. 4A, a picture signal portion 4A (which should be at the black level) of analog picture signal 4 has a level lying between the white and black levels. Accordingly, if the level of signal portion 4A is slightly above the threshold voltage V.sub.ref, the black picture information will be eliminated totally, as shown in FIG. 4B.
In the case where the central pixel P.sub.i,n is white and the other pixels are black, as shown in FIG. 3B, a picture signal portion 4B corresponding to the pixel P.sub.i,n is produced which has a level lying between the white and black levels as shown in FIG. 5A. If the signal level is slightly below the threshold voltage V.sub.ref, the white picture information will be totally eliminated, as shown in FIG. 6B.
Thus, in a binary transformation apparatus in which a picture image is read by an image sensor, as the line density of the picture increases, the contrast of the picture image is lowered, which results in the risk of increasing the possibility of erroneous binary transformations of picture signals.